The present invention relates to a toner for developing an electrostatically charged image in electrophotography, electrostatic recording or electrostatic printing.
Up to this time, an electrostatically charged image formed on a recording medium in electrophotography, electrostatic recording or electrostatic printing has been developed by two main methods, i.e., a wet developing method using a developer comprising a fine dispersion of various pigments or dyes in an insulating liquid or a dry developing method using a finely powdered developer called toner and prepared by dispersing a coloring material in a natural or synthetic resin. Examples of the latter method include cascade development, manual brushing, magnetic brushing, impression development and powder cloud development. The present invention relates to a toner suitable for the dry developing methods.
Up to this time, a toner for developing an electrostatically charged image has been prepared by dispersing a coloring material in a soft polymer by melting, kneading and grinding the obtained polymer containing the coloring material dispersed therein. However, the powder obtained by this process has a very wide particle size distribution so that the powder must be classified prior to the practical use as a toner. Thus, the process itself is disadvantageous in complexity and cost.
Further, the toner prepared by the above process involving a grinding step has edges and small cracks. Therefore, the resulting toner has poor fluidity and when it is stirred in a developing device these edges and small cracks break to generate dust which causes a lowering in the quality of the image, or scumming, thus shortening the life of the image.
On the other hand, several polymerization processes for directly preparing a colored polymer particle not involving a grinding step have been proposed in, for example, Japanese Patent Publication Nos. 10231/1961, 51830/1972 and 14895/1976 and Japanese Patent Laid-Open Nos. 17735/1978, 17736/1978 and 17737/1978.
These processes comprise suspending an oil phase containing a monomer, a polymerization initiator and a coloring material in an aqueous medium and polymerizing the obtained suspension to directly obtain a toner and relate to so-called suspension polymerization.
These processes have advantages in that the obtained toner is spherical and excellent in fluidity and in that the preparation process itself is simple and the cost is low.
However, the toner prepared by these processes has disadvantages in that the properties are highly dependent upon humidity and therefore is poor in humidity resistance and electrostatic chargeability such that the electrostatic chargeability and the maintenance of a charge are insufficient even at ordinary temperature and humidity so as to give a low-quality image.
The reason for the above disadvantages have been researched and, as a result, the following thoughts presented. Since carbon black, which has been uniformly dispersed among monomers at the initiation of suspension polymerization, gathers near the surface of the toner particle during the polymerization, the surface resistance of the obtained toner is lowered, so that the electrostatic chargeability and charge stability of the toner are also lowered, of which the latter is particularly lowered at high humidity.